Process of hardening steel wires



Patented June 11, 1940 UNITED STATES PATENT OFFICE PROCESS OF HARDENING STEEL WIRES Robert Suchy, Bitte'rfeld, Germany, as'signor to I. G. Farbenindustrie Aktlengesellschai't, Franki'ort-on-the-Main, Germany No Drawing.

3 Claims.

temperature of about 950 to 1000 C. and thereupon cooling the soaked article. in the air (air hardening) or preferably passing it through a bath of molten lead heated to a temperature of about 500 C. (lead hardening).

Despite their good mechanical properties, however, steel wires hardened by the foregoing known processes are found to have a poor resistance to corrosion in contact with the atmosphere or with water. i

It has now been ascertained, in accordance with the present invention that the tendency of the wires to corrode is minimised if the quenching of the wires, heated to about 1000? C., takes place in a fused chromate bath and not in air or in a molten lead bath. The fused chromate bath is kept at a temperature of approximately 500 C.

It has been recently ascertained that alkali metal dichromates, when in the molten condition, and particularly when heated to temperatures in excess of about 350 C., are liable to a progressive decomposition, a certain proportion of monochromate being formed together with a sludge substantially consisting of chromium oxide. Such decomposition proceeds until an equilibrium between monochromate and dichromate is attained, the percentage proportion of monochromate necessary to establish such equilibrium being dependent on the temperature at which the chromate melt is maintained. It has therefore been suggested, when using alkali metal chromate baths in the heat treatment of metals, to employ baths consisting of a mixture of dichromate and monochromate in such proportion that the con-' tent of monochromate is at least as great as that corresponding to the equilibrium ratio set up between dichromate and monochromate at the temperature at which the bath is to be used.

In carrying out the present invention the chromate baths employed in the hardening of steel wires are therefore preferably made up of a mixture of alkali metal monochromate and alkali metal dichromate in such proportion that the content of monochromate is at least as great as that prescribed by the equilibrium ratio. For a temperature of about 500 C. the fused saline bath would thus consist of about 630 parts by weight of potassium dichromate and 3'70 parts by weight of potassium monochromate.

The steel wires heated to about 1000 C. and thereupon quenched according to the invention at about 500 C. in a fused saline bath consisting,

Application August 9, 1938, Serial In Germany August 18, 1937 for example, of 63 parts by weight of potassium dichromate and 37 parts by weight of potassium monochromate, acquire the same mechanical properties as wires quenched in air or in a bath of molten lead. Exhaustive tests have shown, however, that in respect of the resistance to corrosion wires quenched in accordance with the invention are far superior to the latter. 1

The following table shows that the loss of material through the formation of rust during the action, over a period of several days, of tap water on wires which werequenched in air or a bath of molten lead after soaking at about 1000 C. is many times as great as that on wires quenched in fused chromate baths in accordance with the present invention.

parts of whic at 500? C.

I claim:

l. A process of hardening steel wires, which comprises heating such wires to temperatures above the critical range and thereafter quenching such wires in a fused saline bath substantially consisting of alkali metal chromate which is maintained at a temperature of approximately 500 C.

2. A process of hardening steel wires, which comprises heating such wires to temperatures above the criticalrange and thereafter quench ing such wires in a fused saline bath maintained at a temperature of approximately 500 C. and substantially consisting of alkali metal dichromate and alkali metal monochromate in such proportions that the monochromate content at least corresponds to the equilibrium ratio be- 'tween monochromate and dichromate at the temperature of the bath.

3. A process of hardening steel wires, which comprises heating such wires to temperatures above the critical range and thereafter quenching such wires in a saline bath consisting of about 630 parts by weightof potassium dichromate and 370 parts by weight of potassium monochromate, such bath being maintained at a temperature of about 500 C.

ROBERT SUCHY. 

